Many attempts have been made to obtain accurate direction measurements by means of a magnetic compass so as to largely eliminate disturbances due to the fact that such compass is located on a large ferromagnetic body such as an armored vehicle or on a battle-tank. Every ferro-magnetic body has a certain permanent magnetization acquired during its manufacture, and, for example with tanks, this differs from one tank to the other even in the same production series.
Furthermore, due to the magnetic field of the earth a temporary magnetization is induced in such ferromagnetic bodies, depending on the position respective the earth magnetic field vectors and the characteristic parameters of the vehicles (to be illustrated with respect to a tank), including the magnetic properties as well as the geometry of the tank including dimensions, permeability, mass and shape as well as angle respective the horizontal plane.
The resulting secondary magnetic field in the vicinity of the tank adds vectorially to the magnetic field of the earth and thus a compass attempting to measure the magnetic horizontal vectors will provide erroneous results.
Attempts to attain suitable corrections of such measurements by polynomial approximations have been made before.
In most existing systems the magnetic flux gates have been installed at close proximity to the main ferromagnetic body, and in specific cases close to the tank turret. Mathetmatic approximations by means of computations based on Fourier series have been attempted, but these were based on a very large number of parameters, requiring large and expensive computation means and complicated computations for a large number of corrective positions. It seems that this computation system is too complicated for larger angles of turret versus hull, including the inclination of the tank versus the horizontal plane.
In order to attain an accuracy suitable for land navigation or target acquisition, an electronic compass should be able to evaluate the perturbations introduced by the ferromagnetic body either through a sufficiently reliable empirical calibration method or through physical modelisation of the perturbation generating body, substract the perturbation from the overall measured horizontal field components, and thus arrive to the corrected azimuth. Electronic compasses that use empiricalmathematical calibration methods for the correction of the measured field components are described in U.S. Pat. Nos. 3,596,069; 3,696,518; 4,414,753; 4,686,772; British Patent No. 591019 and Israeli Patent No. 64237; the present invention, as it will be described below, makes use of the basic property of magnetic fields produced by finite bodies, namely, field decay with increasing distance and evaluates analytically the perturbation by replacing the actual shape of the ferromagnetic bodies with suitable dimensioned closed geometrical structures; moreover, an optimization method is used to find the best position for the magnetic sensors, thus minimizing the perturbation. The analytical modelization is applicable to attain a satisfactory degree of accuracy, owing to the fact that in the configuration of the present invention, the magnetic field sensors are removed from the ferrous body as much as mechanical and other technical constraints permit.
In Israeli patent No. 78889 residual magnetization of the armored vehicle is represented by a correction vector of the magnetization, and this is computed on the basis of Fourier series. This computation is based on the angle between the main body of the tank and its turret. The correction is based only on the relative angle of the turret versus the main body and magnetic measurements. It seems that the said patent requires calibration based on 3264 parameters, for 16 positions only of the turret/main body.
The close proximity of the magnetic sensor respective the main iron body causes serious problems which are overcome by the configuration of the measurement system of the present invention.
The present invention relates to an electronic magnetic compass system for determining the magnetic heading (azimuth) of a body made of ferro-magnetic material, such as an armored vehicle, and especially a tank. According to the present invention the main parameters of the measured system, such as position of turret versus main body of the tank, the inclination of the tank versus the horizon etc. are taken into consideration, resulting in a comparatively high accuracy of the measurement.
The electronic compass system of the present invention comprises the following main components:
a biaxial fluxgate magnetometer located on an antenna-like structure at a predetermined position and at a certain distance above the upper surface of the turret;
two inclinometer sensor means for the determination of the angle of the armored vehicle respective the horizontal plane;
means for measuring the angle between the hull and the turret of a tank;
suitable interface means with microprocessor for azimuth computation; a memory device storing an azimuth correction algorithm, based on the modelization of the two main components, hull and turret by means of two ellipsoids, each tank or armored vehicle being calibrated and the measured parameters being stored for future computations, display means being provided at desired locations.
The location of the fluxgate magnetometer at a distance of about 1.5 meters above the turret greatly reduces disturbances by the magnetic fields of the ferromagnetic bodies. The presence of a large ferromagnetic body in the magnetic field of the earth causes serious deviations in the measured magnetic field, and these are corrected by the system of the invention, in combination with the algorithm used for the computation.
Generally an accuracy of .+-.3.degree. or better can be attained, which is adequate for most requirements.
According to the present invention, a limited number of parameters is required for the computations. For most uses the use of about 8 to 14 parameters was found to be adequate.
It is a further feature of the present invention that there is used a miniaturized biaxial fluxgate magnetometer, which can be positioned in an antenna-like structure above the turret.
The corrective system of the present invention is based on two ellipsoids of different size and shape, one representing the hull of the tank and the other its turret, such ellipsoids, of elongated shape, and made of a ferromagnetic material of suitable permeability, and which are rotatable versus each other (representing the rotation of the hull/turret system), in combination with a suitable determination of the inclination of the tank versus the horizon, and taking this into consideration in the computations, allows a rather simple and speedy determination of the required correction factor, providing a direct readout of the heading of the axis of the turret or of the hull axis if desired.
The computation is based on the assumption that the ellipsoids undergo magnetization and disturb the measured magnetic field. The model determines with adequate accuracy and by means of comparatively simple computations the disturbances resulting along each of the three axes of the ellipsoids and thus also compensates for any angular inclination of the tank. It is based with armored vehicles on 8 parameters only, and with heavy battle-tanks on the use of only 14 parameters. Each individual tank must be calibrated separately, and the results are stored in suitable memory means. It is generally adequate to use a 16 bit microcomputer with a memory of the order of 10K bytes.
According to the present invention a miniaturized biaxial flux-gate magnetometer is used which is positioned well above the upper surface of the turret, and preferably at a distance of about 1.5 meters above such surface. This distance greatly reduces magnetic disturbance due to the heavy ferromagnetic body and facilitates corrections of adequate accuracy.
The system of the invention comprises two inclometer sensors for determining the angle of the two main axes of the tank versus the horizontal and means for measuring the angles between the axes of the hull and the turret of the tank, in conjunction with signal processing and evaluation means, based on calibrating data stored in memory means of the system, and display means for indicating the corrected angle of heading of the tank.
The computations are based on correction algorithms and computations are made for both a certain type of vehicle (such as a certain tank model) and also for each individual vehicle.
The optimum location of the magnetometer and of field sensors is calculated and this determines their location on the armored vehicle.